Measuring and operating circuit for a coriolis-type mass flowmeter

ABSTRACT

According to the invention, a digital processor (DSP) is provided in a measuring and operating circuit for a Coriolis-type mass flowmeter. From the vibration sensor signals, said digital processor evaluates only the differential signal (D) and the one sensor signal (S 1 ). The in-phase component (I) and the quadrature components (Q) are determined for the differential signal (D), and the amplification of the second sensor signal (S 2 ) is controlled in such a manner that the in-phase component (I) vanishes. The mass flow rate (m) is determined from the quadrature component (Q).

TECHNICAL FIELD

This invention relates to a measuring and operating circuit for aCoriolis mass flowmeter.

BACKGROUND OF THE INVENTION

Coriolis mass flowmeters are widely used to determine the mass flow rateof a fluid in a section of pipe. The fluid passes through at least onevibrating flow tube. In most Coriolis mass flowmeters, a vibrationexciter and two vibration sensors are mounted on the flow tube. The flowtube and the fluid together form a vibratory system which is normallyexcited at its resonance frequency. The resonance frequency depends,among other things, on the material and dimensions of the flow tube. Italso varies with the density of the flowing fluid. In some cases, theflow tube is excited not at the resonance frequency, but at an adjacentfrequency.

The two vibration sensors sense the vibratory motion of the flow tube attwo locations spaced a given distance apart in the direction of fluidflow, and convert this vibratory motion into sensor signals. Both sensorsignals have the same frequency as the vibratory motion of the flowtube, but they are out of phase.

The phase difference is a measure of the mass flow rate. In a measuringsubcircuit, the sensor signals are evaluated and converted to a signalproportional to the mass flow rate of the fluid. Aside from the massflow rate, further properties of the fluid, e.g. its density, can bedetermined. This is accomplished, for example, by evaluating thefrequency of the vibratory motion of the flow tube.

U.S. Pat. No. 4,801,897 describes an excitation subcircuit which isconstructed in the manner of an analog phase-locked loop. In thatcircuit, the excitation frequency adjusts itself automatically to theresonance frequency of the vibratory system even during variations influid density.

The prior-art measuring circuits use either analog techniques, asdescribed in EP-A 698 783 or U.S. Pat. No. 4,895,030, for example, ordigital techniques, as described in EP-A 702 212 or U.S. Pat. No.5,429,002, for example.

EP-A 698 783 discloses a measuring circuit comprising an analog controlloop which regulates the two sensor signals at the same amplitude.

EP-A 866 319 discloses a further measuring and operating circuit. Inthis circuit, the two sensor signals are amplified before beingprocessed, with one of the amplifiers having a variable gain.

In a digital processor, the sum and difference of the two sensor signalsas well as one of the sensors signals are evaluated.

For the accuracy of the measurement it is essential that after theiramplification, the two sensor signals have the same amplitude. Theamplitude regulator required for this purpose evaluates the sum anddifference of the two sensor signals.

For the actual determination of the mass flow rate, in addition to thedifference signal, the signal from one of the two sensors is needed.

Altogether, in this circuit, three analog vibration signals are formedand then processed in an arithmetic unit. For each vibration signal, atleast one A/D converter is necessary in the arithmetic unit.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an operating circuit for aCoriolis mass flowmeter in which fewer vibration signals have to beformed and evaluated and which nevertheless has sufficient accuracy andis easy and inexpensive to implement.

This object is attained by a measuring and operating circuit for aCoriolis mass flowmeter comprising a transducer assembly with at leastone flow tube on which a first and a second vibration sensor, spaced agiven distance apart in the direction of fluid flow, and a vibrationexciter are mounted, the measuring and operating circuit comprising: afirst amplifier, which is connected to the first vibration sensor; asecond amplifier, which is connected to the second vibration sensor; afirst A/D converter for generating a vibration signal S1 proportional tothe output signal of the first vibration sensor, which is connected tothe first amplifier; a difference stage having its two inputs connectedto the first amplifier and the second amplifier, respectively; a secondA/D converter, following the difference stage, for generating adifference signal D proportional to the difference of the amplifiedoutput signals from the first and second vibration sensors; and adigital processor which, of the vibration sensor signals, evaluates onlythe difference signal D and the sensor signal S1, and which performs thefollowing steps:

a) Determining the amplitude AMS1 of the sensor signal S1

b) Determining the in-phase component I and the quadrature components Qof the difference signal D with respect to the sensor signal S1 as areference signal

c) Controlling the gain of the second amplifier in such a way that thein-phase component I of the difference signal disappears

d) Calculating the mass flow rate via the remaining quadrature componentQ according to the formula

m˜Q/(AMS1*f).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more apparent from the following descriptionof an embodiment when taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic representation of a transducer assembly of aCoriolis mass flowmeter;

FIG. 2 is a block diagram of a measuring and operating circuit for aCoriolis mass flowmeter; and

FIG. 3 is a block diagram illustrating individual steps of the methodcarried out in the measuring circuit of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows schematically a transducer assembly 1 for a Coriolis massflowmeter. Transducer assembly 1 is installed in a pipe (not shown)carrying a fluid F whose mass flow rate is one of the quantities ofinterest. The connection with the pipe is made by means of two flanges2, 3.

Transducer assembly 1 is a single straight flow tube 4, which is fixedto flange 2 at the inlet end via an end plate 13 and at the outlet endvia an end plate 14.

The use of the measuring and operating circuit according to theinvention is not limited to this specific transducer assembly 1 with asingle straight flow tube. The circuit can also be employed inconjunction with various conventional transducer assemblies, e.g. withtransducer assemblies having a flow tube with a cantilever mass asdescribed in EP 97 81 0559, for example, with transducer assemblieshaving a single curved flow tube (EP 96 10 9242), and with transducerassemblies having two parallel straight or curved flow tubes (U.S. Pat.Nos. 4,793,191 and 4,127,028, respectively),

The flanges 2, 3 and the end plates are fixed to or in a support tube15.

To generate the flow-tube vibration, a vibration exciter 16 is mountedon flow tube 4 midway between the two end plates 13, 14. Vibrationexciter 16 may be, for instance, an electromagnetic driving mechanismconsisting of a permanent magnet 161 and a coil 162.

Coil 162 is fixed to support tube 15, and permanent magnet 161 to flowtube 4.

Via the current flowing in coil 162, the amplitude and frequency of theflexural vibration of flow tube 4, a vibration performed in the plane ofthe paper, can be controlled.

Also in the plane of the paper, Coriolis forces occur, as a result ofwhich not all the points along flow tube 4 vibrate in phase. Thevibratory motion of flow tube 4 is sensed by means of two vibrationsensors 17 and 18 which are mounted on support tube 15 symmetricallywith respect to vibration exciter 16. Vibration sensors 17 and 18 maybe, for instance, electromagnetic transducers similar in construction tothe permanent magnet and coil assembly of vibration exciter 16. The twopermanent magnets 171, 181 are fixed to flow tube 4, and the two coils172, 182 to support tube 15. The motion of flow tube 4 causes voltagesto be induced in coils 172 and 182 via magnets 171 and 181,respectively. These voltages are picked off as analog sensor signals X17and X18, respectively.

A Coriolis mass flowmeter, as a rule, consists of a transducer assemblyand an associated measuring and operating circuit. FIG. 2 shows a blockdiagram of a measuring and operating circuit associated with transducerassembly 1. The functions of this circuit include evaluating the sensorsignals and controlling the excitation of vibrations.

The two sensor signals X17 and X18 are applied to a first amplifier V1and a second amplifier V2, respectively. At least the gain of amplifierV2 is variable.

Amplifier V1 is connected to a first A/D converter AW1 and, in paralleltherewith, to one input of a difference stage D1.

Amplifier V2 is connected to a further input of difference stage D1. Theoutput of difference stage D1 is coupled to a second A/D converter AW2.

The two outputs of A/D converters AW1 and AW2 provide the sensor signalS1 and the difference signal D, respectively, in digitized form. Bothoutputs are connected to a digital processor DSP.

The first amplifier V1 and the first A/D converter AW1 form a firstvibration signal path SW1. The second amplifier V2, the difference stageD1, and the second A/D converter AW2 form a second vibration signal pathSW2.

Accordingly, of the two signals from the vibration sensors, only thedigital sensor signal S1 and the digital difference signal D are fed todigital processor DSP over two vibration signal paths SW1 and SW2,respectively.

Digital processor DSP provides at a first output A1 a signalproportional to the mass flow rate m. A second output A2 of digitalprocessor DSP, which provides a gain control signal VS, is connected toan input of a D/A converter DW1, whose output is coupled to amplifierV2. By means of the gain control signal VS, the gain of the secondamplifier V2 is adjusted.

A third output A3 provides a signal which controls the excitationcurrent I_(err) for exciting vibrations of the flow tube.

FIG. 3 shows schematically the individual steps to determine the massflow rate m.

Step a): Determining the amplitude AMS1 of sensor signal S1

To determine the amplitude of the digital sensor signal S1, the latteris multiplied by a standard sine-wave signal SE and a standardcosine-wave signal CE, and the signals obtained are filtered withlow-pass filters TP1 and TP2, respectively. The low-pass filters provideamplitude values a and b which specify the shares of the sensor signalS1 according to the two standard signals SE and CE. Extracting the rootof the sum of squares a²+b² gives the amplitude AMS1 of the sensorsignal S1, measured in a coordinate system which is spread by the twostandard signals SE and CE.

Step b); Determining the in-phase component I and the quadraturecomponents Q of the difference signal D with respect to the sensorsignal S1 as a reference signal

The difference signal D is multiplied by the sensor signal S1 and thenfiltered in a low-pass filter TP4 to obtain the in-phase component I ofthe difference signal D.

In addition, the difference signal D is multiplied by the sensor signalS1 after the latter has been shifted in phase by 90°, and the signalobtained is filtered in a low-pass filter TP3 to obtain the quadraturecomponent Q of the difference signal D.

Step c): Controlling the gain of the second amplifier such that thein-phase component I of the difference signal D disappears

The in-phase component I of the difference signal D is fed to acontroller R which provides a gain control signal VS with which the gainof amplifier V2 is so controlled that the component I disappears.

When the in-phase component I of the difference signal D disappears, thetwo signal amplitudes at the outputs of the two amplifiers V1 and V2 arenearly equal. The difference between the signal amplitudes decreaseswith decreasing phase difference between the sensor signals X17 and X18.

Step d): Calculating the mass flow rate via the remaining quadraturecomponent Q according to the formula

m˜Q/(AMS1*f)

From the values of the quadrature component Q and the amplitude sodetermined, the mass flow rate m is determined according to the formula

m˜Q/(AMS1*f).

The frequency f is provided by a generator G.

In generator G, the two standard signals SE and CE are generateddigitally.

The standard cosine-wave signal CE is multiplied by a variable amplitudeAMP to obtain the signal U_(err). The signal U_(err) is used to controla driver circuit TR which delivers the excitation current for vibrationexciter 16.

To excite the vibratory system exactly at its resonance frequency,digital processor DSP determines the phase difference dφ between theexcitation signal U_(err) and the response function of the system, thesensor signal S1. The frequency f of the standard signals SE and CE isso controlled that the phase difference dφ becomes zero. In that case,the exciting force is in phase with the vibration velocity of flow tube4.

What is claimed is:
 1. A measuring and operating circuit for a Coriolismass flowmeter, comprising: a transducer assembly including: at leastone flow tube; at least a first and second vibration sensor, spaced agiven distance apart in the direction of fluid flow along said at leaseone flow tube; and a vibration exciter mounted on said at least one flowtube; said measuring and operating circuit comprising: a first amplifierfor amplifying the output signal of said first vibration sensor; asecond amplifier for amplifying the output signal of said secondvibration sensor; a first A/D converter for generating a vibrationsignal proportional to the output signal of said first vibration sensor;difference stage means connected to said first amplifier and said secondamplifier for generating a difference signal proportional to thedifference of the amplified output signals of said first amplifier andsaid second amplifier; a second A/D converter for receiving saiddifference signal; and a digital processor for receiving the output offirst A/D converter and said second A/D converter to thereby evaluatesaid vibration signal generated by said first A/D converter and saiddifference signal from said second A/D converter.
 2. The measuring andoperating circuit as defined in claim 1, wherein said digital processor:determines the amplitude of the amplified output signal of said firstvibration sensor; determined an in-phase component and quadraturecomponents of said difference signal with respect to the amplifiedoutput signal of said first vibration sensor as a reference signal;controls the gain of said second amplifier such that said in-phasecomponent disappears; and calculates the mass flow rate via theremaining quadrature component according to the formula m˜Q/(AMS1*f)where: m represents the mass flow rate; Q represents the quadraturecomponents; AMS1 represents the amplitude of the amplified output signalof said first vibration sensor; and f represents frequency.